U.S. patent number 5,701,898 [Application Number 08/651,022] was granted by the patent office on 1997-12-30 for method and system for doppler ultrasound measurement of blood flow.
This patent grant is currently assigned to The United States of America as represented by the Department of Health. Invention is credited to Dan R. Adam, Michael Jones, Kenneth M. Kempner, Eben E. Tucker, Mark A. Vivino.
United States Patent |
5,701,898 |
Adam , et al. |
December 30, 1997 |
Method and system for Doppler ultrasound measurement of blood
flow
Abstract
A method and system for providing Doppler data that is corrected
for misalignment between the flow direction within a vessel and the
beam orientation of the ultrasound probe. A conventional ultrasonic
Doppler color mapping system is adapted to include a system to
measure and record the free space position and orientation of the
ultrasonic probe, as well as a system to generate and record the
experiment time. A set of 2D image planes is acquired, each plane
labelled with time, position, and orientation data. A structural
representation according to the acquired data is used to determine
the flow direction for the imaged vessel structure. Based on this
identified flow direction, and the orientation and position
information for each acquired 2D slice, the 2D Doppler signals are
appropriately transformed into corrected velocity values. These
corrected velocity values may then be used to construct a 3D flow
field as a function of time.
Inventors: |
Adam; Dan R. (Haifa,
IL), Kempner; Kenneth M. (Bethesda, MD), Vivino;
Mark A. (Bethesda, MD), Tucker; Eben E. (Silver Spring,
MD), Jones; Michael (Bethesda, MD) |
Assignee: |
The United States of America as
represented by the Department of Health (Washington,
DC)
|
Family
ID: |
23160291 |
Appl.
No.: |
08/651,022 |
Filed: |
May 21, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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300718 |
Sep 2, 1994 |
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Current U.S.
Class: |
600/454;
600/455 |
Current CPC
Class: |
G01S
15/8979 (20130101); A61B 8/4254 (20130101); A61B
8/483 (20130101); G01S 15/8984 (20130101); G01S
15/8993 (20130101); A61B 8/06 (20130101); G01S
7/5205 (20130101); A61B 5/352 (20210101); A61B
8/543 (20130101); G01S 15/899 (20130101) |
Current International
Class: |
A61B
8/06 (20060101); G01S 15/89 (20060101); G01S
15/00 (20060101); A61B 5/0456 (20060101); A61B
5/0452 (20060101); G01S 7/52 (20060101); A61B
008/06 () |
Field of
Search: |
;128/660.04,660.05,660.07-660.1 ;364/413.25 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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CR: Dynamic Three-Dimensional Echocardiographic Reconstruction of
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In Patients. AM Heart J 1982; 103: 1056-1065. .
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Heart J 1983; 50:438-442. .
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Martin RW, Bashein G: Measurement of stroke Volume with
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Kuroda T, Kinter TM, Seward JB, Yanagi H, Greenleaf JF: Accuracy of
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Echocardiographic Probe: In Vitrp Experiment. J. Am Soc
Echocardiogr 1991; 4:475-484. .
Martin RW, Bashein G, Nessly ML, Sheehan FH: Methodology for
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Transesophageal Echocardiograms. Ultrasound in Med & Biol 1993;
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Picot PA, Rickey DW, Mitchell R, Rankin RN, Fenster A:
Three-Dimensional Colour Doppler Imaging. Ultrasound in Medicine
and Biolog, vol. 19, No. 2, 1963, Official Journal of the World
Federation for Ultrasound in Medicine and Biology; pp. 95-104.
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Kitney RI, Moura L, Straughan K: 3-D Visualization Of Arterial
Structures Using Ultrasound and Voxel Modeling. Int J Card Imaging
1989; 4:135-143. .
Potkin BN, Bartorelli AL, Gessert JM, Neville RF, Almagor Y,
Roberts WC, Leon MB: Coronary Artery Imaging With Intravascular
High-Frequency Ultrasound. Circulation 1990; 81: 155-1585. .
Nissen SE, Grines CL, Gurley JC, Sublett K, Haynie D, Diaz C, Booth
DC, DeMaria AN: Application Of A New Phased-Array Ultrasound
Imaging Catheter in the Assessment of Vascular Dimensions.
Circulation 1990; 81:660-666. .
Rosenfield K, Losordo DW, Ramaswamy K, Pastore JO, Langevin RE,
Razvi S, Kosowsk BD, Isner JM: Three-Dimensional Reconstruction of
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Two-Dimensional Intravascular Ultrasound Examination. Circulation
1991; 84:1938-1956..
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Primary Examiner: Jaworski; Francis
Attorney, Agent or Firm: Morgan & Finnegan, LLP
Parent Case Text
This is a continuation of application Ser. No. 08/300,718, filed on
Sep. 2, 1994 and now abandoned.
Claims
We claim:
1. A Doppler ultrasound apparatus for measuring flow in a localized
region within a vessel, said flow having an associated velocity,
said apparatus comprising:
an ultrasound probe for generating and detecting an incident and
reflected ultrasound signal, respectively, said incident and
reflected ultrasound signal including a Doppler ultrasound
signal;
a position and orientation system which generates an orientation
and position signal for said ultrasound probe; and
a processor which provides a corrected ultrasound signal
representing said flow, based on said orientation and position
signal, and on said ultrasound signal to generate a
three-dimensional representation of said vessel for determining a
three-dimensional flow direction for said localized region within
the vessel;
wherein said corrected ultrasound signal compensates for errors in
measuring said flow that are due to angular misalignment of said
ultrasound probe relative to said velocity of said flow in said
localized region.
2. The apparatus according to claim 1, wherein said ultrasound
signal includes Doppler information for a plurality of localized
regions within a two dimensional region scanned by said ultrasound
probe according to a plurality of scanning lines.
3. The apparatus according to claim 2, wherein said ultrasound
probe scans a plurality of two dimensional regions.
4. The apparatus according to claim 3, wherein said plurality of
two dimensional regions are scanned as a function of time, and said
apparatus provides a time dependent three dimensional flow
image.
5. The apparatus according to claim 1, wherein said ultrasound
signal includes a structural ultrasound signal.
6. The apparatus according to claim 1, wherein said ultrasound
signal includes a time series of structural information and Doppler
information.
7. The apparatus according to claim 6, further comprising a means
for acquiring said ultrasound signal in a known time relationship
with the cardiac cycle of a body observed by the Doppler ultrasound
apparatus.
8. The apparatus according to claim 7, wherein said ultrasound
signal is acquired over a plurality of cardiac cycles, said
processor separately processing data sets of said time series of
structural information and Doppler information according to the
acquisition time for each data set relative to the cardiac
cycle.
9. The apparatus according to claim 7, wherein said corrected
ultrasound signal represents a corrected flow velocity.
10. The apparatus according to claim 1, wherein said position and
orientation system includes a transmitter and a receiver, one of
said transmitter and receiver fixed in space relative to other of
said transmitter and receiver, one of said transmitter and receiver
fixably mounted to said ultrasound probe, said position and
orientation system including a processor for generating an
orientation matrix which relates the spatial transformation between
the respective coordinate systems of the transmitter and the
receiver, thereby providing said orientation and position
signal.
11. The apparatus according to claim 1, wherein said processor
provides said corrected ultrasound signal based on determining a
smooth and continuously changeable central axis for flow according
to structural information in said Doppler ultrasound signal.
12. A Doppler ultrasound apparatus for measuring flow velocity in a
localized region within a vessel using Doppler ultrasound, said
apparatus comprising:
an ultrasound probe for generating and detecting an incident and
reflected ultrasound signal, respectively, said incident and
reflected ultrasound signal including a Doppler ultrasound
signal;
means for generating an orientation and position signal for said
ultrasound probe; and
means for processing said orientation and position signal, and said
ultrasound signal into a corrected ultrasound signal representing
said flow velocity based on generating a three-dimensional
representation of said vessel for determining a three-dimensional
flow direction for said localized region within the vessel;
wherein said corrected ultrasound signal compensates for errors in
measuring said flow that are due to angular misalignment of said
ultrasound probe relative to said flow velocity in said localized
region.
13. The apparatus according to claim 12, further comprising means
for generating a time code associated with the time of detecting
said ultrasound signal, and wherein said ultrasound signal includes
a time series of image data corresponding to a two dimensional
region scanned by said ultrasound probe according to a plurality of
scan lines.
14. The apparatus according to claim 13, wherein said ultrasound
signal includes a image data for a plurality of two dimensional
regions, and further comprising means for generating a three
dimensional time dependent representation of said flow according to
said corrected ultrasound signal.
15. A method for measuring a flow in a localized region within a
vessel using a Doppler ultrasound apparatus, said flow having an
associated velocity, said method comprising the steps of:
generating and detecting an incident and reflected ultrasound
signal, respectively, using an ultrasound probe, said incident and
reflected ultrasound signal including a Doppler ultrasound
signal;
generating an orientation and position signal for said ultrasound
probe; and
processing said orientation and position signal, and said
ultrasound signal into a corrected ultrasound signal representing
said flow by generating a three-dimensional representation of said
vessel for determining a three-dimensional flow direction for said
localized region within the vessel;
wherein said corrected ultrasound signal compensates for errors in
measuring said flow that are due to angular misalignment of said
ultrasound probe relative to said velocity of said flow in said
localized region.
16. The method according to claim 15, wherein said processing step
includes determining a smooth and continuously changeable central
axis for flow according to structural information in said Doppler
ultrasound signal.
17. The method according to claim 15, further comprising the step
of generating a three dimensional flow image according to said
corrected ultrasound signal.
18. The method according to claim 17, wherein said three
dimensional flow image corresponds to flow at a particular time
with respect to the cardiac cycle of a body observed by the Doppler
ultrasound apparatus.
19. The method according to claim 17, further comprising the step
of generating said three dimensional flow image as a function of
time.
20. The apparatus according to claim 1, wherein said ultrasound
signal includes a structural ultrasound signal in addition to the
Doppler ultrasound signal, and wherein said processor provides said
corrected ultrasound signal based on determining a smooth and
continuously changeable central axis for flow according to the
structural ultrasound signal.
Description
TECHNICAL FIELD
The present invention relates generally to ultrasound imaging and,
more particularly, to a method and system for three dimensional
(3D) color Doppler ultrasound imaging of measured blood flow.
BACKGROUND OF THE INVENTION
In the past, various methods have been applied to the problem of
reconstructing 3D anatomical structures within the cardiovascular
system, from 2D ultrasound images (e.g. the contour of the entire
heart, or of the left ventricle). Multiple images of structural
cross-sections of the heart have been taken at various orientations
in order to reconstruct its shape. Since the introduction of
esophageal probes, the better image quality and the nearly fixed
location of the ultrasound transducer within the body have made
these systems well suited for 3D reconstruction. The accuracy for
volume measurements has also been demonstrated. In addition, the
further development of miniaturized probes has allowed
intravascular generation of 2D images and therefore, the 3D
reconstruction and visualization of arterial structures, including
coronary arteries. In all these cases, the accuracy and the sources
of error could be studied by the use of an anatomical phantom.
These methods are slowly being adapted for flow measurements. There
is clearly a clinical demand for measurement of the flow velocity
pattern across the flow field. In the study and development of
artificial cardiac valves, quantitative assessment of the 3D flow
jets through and around have valve is required. Blood volume and
its velocity in the artery have significant effects on transport of
metabolic constituents and exchange of substances from blood to the
endothelial cells. Also, the velocity distribution across the
artery is assumed to be one of the determinants of endothelial
layer damage and arterial stenosis. The large shear stresses
associated with high velocities cause shear-induced platelet
aggregation.
Different modalities have been tested for the determination of 3D
flow velocity patterns--Ultrasound, fast Computed Tomography, and
Magnetic Resonance Imaging (MRI). Various approaches have been
taken to allow flow velocity mapping. Validations of the MRI
measurements versus flow phantoms and ultrasound have been
demonstrated. The spatial resolution of MRI flow velocity
measurement is, however, too low, the acquisition time is longer
than the cardiac cycle, and the cost is prohibitive. Similarly, the
validations performed for this method demonstrate the inability to
obtain accurate, time dependent flow velocity profile
determination.
Doppler ultrasound mapping of flow velocities is now available for
both external and esophageal scanning transducers. Both produce 2D
images of intra-ventricular and vascular flow, but the accuracy of
the measurements is too low for quantitative evaluation and 3D
reconstruction. The application of 2D color flow Doppler techniques
to the detection and diagnosis of cardiac and vascular
abnormalities is also limited by the inaccurate color flow data
provided due to the angular error between the orientation of the
Doppler beam and the central axis of the flow field. Intravascular
probes may produce superior images, but their position within the
vessel substantially affects and distorts the 3D pattern of flow.
Several groups are concentrating on the development of new probe
technology, to acquire flow and structural information, directly in
3D.
There is a need, therefore, for development and improvement of the
currently available ultrasound measurement, processing and display
techniques for providing accurate Doppler information.
SUMMARY OF THE INVENTION
The present invention provides a method and system for providing
corrected Doppler data which is not limited by the disadvantages of
the prior art. The invention involves the application of ultrasonic
imaging, and particularly Doppler imaging, to provide an accurate
representation of blood flow by correcting for the effect of the
angle between the flow direction (e.g., the orientation of an
imaged artery) and the beam orientation of the ultrasound
probe.
A conventional ultrasonic Doppler color mapping system is adapted
to include a system to measure and record the free space position
and orientation of the ultrasonic probe, as well as a system to
generate and record the image acquisition time. Preferably, the
position and measurement system is an electromagnetic
position/orientation system. Using this system, a set of 2D image
planes is acquired, each plane labelled with time, position, and
orientation data. Using conventional techniques, an approximation
of a 3D structural representation is generated from the acquired
data. This 3D reconstruction may be displayed on a monitor which
permits a user to define the flow direction (e.g., longitudinal
axis of an artery) for the imaged structure or, alternatively,
methods may be used to extract the structural information, as well
as the flow direction, directly from the 3D reconstruction. Based
on this identified flow direction, and the orientation and position
information for each acquired 2D slice, the 2D Doppler signals are
appropriately transformed into corrected velocity values according
to the Doppler look angle. These corrected velocity values may then
be used to construct a 3D flow field, preferably represented as a
color encoded flow field, which in turn may be superimposed onto
the 3D structural representation and displayed according to known
methods for representing 3D information. Preferably, the time
information is used to provide the corrected Doppler information as
a function of time.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional aspects, features, and advantages of the invention will
be understood and will become more readily apparent when the
invention is considered in the light of the following description
made in conjunction with the accompanying drawings, wherein:
FIG. 1 is a schematic representation of an ultrasound probe
oriented transversely and longitudinally with respect to a vessel,
and of a position/orientation transmitter and receiver, in
accordance with the present invention;
FIG. 2 is a functional block diagram of a system which employs
conventional components to provide data acquisition in accordance
with the present invention; and
FIG. 3 is a functional block diagram of a system which employs
conventional components to provide data processing in accordance
with the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The method and system of the present invention involves the
accurate measurement of time-tagged data describing the position
and orientation of a 2D Doppler ultrasound probe during the imaging
procedure. In this manner, it is possible to generate a set of 2D
image planes, labelled with time, position, and orientation data.
This provides sufficient information to allow a 3D reconstruction
of the imaged structures, as well as the color encoded flow fields.
In addition, in accordance with the present invention, the 2D color
flow data is processed to provide a corrected color flow map.
Uncorrected color flow maps represent velocities in the direction
of the ultrasound transducer's axis. The corrected 2D color flow
images map the flow velocity field in the direction of the vessel's
centerline axis.
The relationship between the flow direction and the orientation of
the ultrasonic probe is illustrated in FIG. 1, which schematically
illustrates a vessel 10 and an ultrasonic probe 12, which is shown
orientated in two separate positions with respect to the vessel 10.
Preferably, the ultrasonic probe 12 is a conventional linear array
probe which scans a sector in a plane by steering the direction of
the ultrasonic beam according to phase array principles. In a first
position A, the ultrasonic probe 12 is shown oriented such that the
imaged sector is substantially in the same plane as the
longitudinal axis (e.g., flow direction) of the vessel 10, while in
position B the ultrasonic probe 12 is shown oriented such that the
imaged sector is transverse to the longitudinal axis for the vessel
10. In position B, the intersection of the image plane with the
vessel is represented by image slice 16. It may be appreciated that
accurately measuring flow velocity via Doppler imaging according to
position B requires that the angle between the plane of the image
slice 16 (e.g., characterized by vector v") and the flow direction,
referred to as the Doppler look angle, should be zero radians. That
is, the plane of the image slice 16 should be coincident with the
flow direction. It is understood, however, that as a practical
matter this condition may not be achieved by a probe that is
external to the vessel, and moreover, that according to the prior
art the Doppler look angle itself may not be ascertained. The
present invention provides for measuring the flow velocity of blood
in a vessel, and particularly for measuring the 3D color Doppler
image of blood flow in a vessel as a function of time, in
accordance with a method and system which corrects the acquired
data for the effects of the Doppler look angle, thereby providing
accurate velocity flow profiles.
In accordance with the practicing the present invention, FIG. 2 and
FIG. 3, by way of example, schematically depict in block diagram
form two complementary, microprocessor-based systems that may be
employed to accomplish the data collection and analysis tasks. FIG.
2 shows a 3D Doppler Image Acquisition System (3D-DIAS) for use in
acquiring the ultrasound information, and FIG. 3 shows a 3D Doppler
Image Processing system (3D-DIPS) for use in separate image
processing of the acquired data. It is understood that these two
systems are shown as a practical way of practicing the present
invention based on commercially available components. As may be
appreciated by one skilled in the art, and as will be discussed
further hereinbelow, the systems shown may be adapted according to
other commercially available components as well as according to
components that may be designed and constructed by one skilled in
the art in light of the present invention to simplify or otherwise
optimize the overall system design with respect to practicing the
present invention. Preferably, the overall system and method of the
present invention may be included into the hardware and software of
a commercial ultrasound system to provide corrected color Doppler
images as one of the modes available at the bedside in essentially
real time.
The 3D-DIAS system shown in FIG. 2 consists of the components
necessary to generate and record experiment time, spatial position
and orientation of the 2D ultrasound probe 12, and image
information. In accordance with a preferred embodiment, the
ultrasound probe is operated such that structural image information
is acquired in an interleaved manner with respect to Doppler image
information (e.g., duplex mode). Time-of-day and frame number data
are generated with SMPTE longitudinal time code generator 22 (e.g.,
Model TRG-50PC, Horita Company, Mission Viejo, Calif., U.S.A.).
This device outputs time code continuously as an audio signal on an
audio channel of a S-VHS video recorder 24, and via an RS-232
digital data stream to a 486-based, 50 MHz, data logging and
control microcomputer 26 (e.g., Model 450DE, Dell Computer
Corporation, Austin, Tex., U.S.A.). This microcomputer also
collects and logs data from the position/orientation measurement
system 30.
Preferably, the position/orientation measurement system 30 is an
electromagnetic position/orientation, measurement system (e.g., A
Flock of Birds, Ascension Technology Corporation, Buffington, Vt.,
U.S.A.) which allows the determination of position and spatial
orientation of the ultrasound probe 12. This system utilizes a
source antenna as a transmitter 14, and a sensor antenna as a
receiver 16. The receiver 16 is mounted to the ultrasonic probe 12,
while the transmitter 14 is mounted (e.g., on an acrylic shelf)
within the vicinity of the spatial region where the ultrasonic
probe 12 is used during scanning. Both the transmitter 14 and the
receiver 16 contain three orthogonal coils, to generate and detect
a quasi-static magnetic field, respectively. With the mentioned
commercial position/orientation measurement system,
position/orientation determinations can be made at a rate
adjustable between 12 and 144 measurements per second. According to
an embodiment of the present invention, the report rate within
3D-DIAS system is determined independently, and is equal to video
frame rate of the S VHS video recorder 24.
A floating-point processor, within an electronics module 32 of the
position/orientation measurement system 30 computes the Position of
the receiver 16, in cartesian coordinates, relative to the
transmitter 14 (see FIG. 1). In addition, the system computes an
Orientation Matrix, which describes the spatial transformation
between the respective coordinate systems of the transmitter 14 and
receiver 12. This Orientation Matrix is a 3.times.3 matrix whose
elements are functions of the Euler Angles describing the rotations
around the axes of the reference coordinate system. An interactive
Time, Position and Orientation Measurement System (TPOMS) software
environment may be employed on the microcomputer 26 to provide
real-time instrument control and data collection. Such a program
may be written in Basic (e.g., QuickBasic, V. 4.5, Microsoft
Corporation, Redmond, Wash., U.S.A.), and uses the efficient RS-232
Communications, and ASCII character manipulation capabilities
provided by the QuickBasic Compiler. Output from TPOMS is a File
containing the SMPTE time cede, and transducer position and
orientation data for each image frame in a study. By way of
example, a study may be composed of multiple 10-second image
collection sequences of 301 image frames each.
The images obtained during a study are captured in analog form on S
VHS video recorder 24 (e.g., Model AG-7350, Panasonic Corporation,
Secaucus, N.J., U.S.A.) and in digital form on a magneto-optical
disc drive 28 (e.g., Model 650/A, Hewlett Packard Company, Palo
Alto, Calif., U.S.A.). Both of these devices are typically included
as components of the Cardiac Doppler Ultrasound System 36 (e.g.,
Sonos, Model 1500, Hewlett Packard Company, Andover, Mass.,
U.S.A.), when employed in laboratory studies. It is understood,
that in accordance with commercially available components, the
Cardiac Doppler Ultrasound System 36 does not provide for storing a
time code for each image frame on the magneto-optical disc. One
skilled in the art, however, recognizes that it is possible to
adapt the Cardiac Ultrasound System 36 to include a time code with
each stored image. Further, one skilled in the art recognizes that
an alternative way of providing this function is to provide the
digital output of the SMPTE longitudinal time code generator 22 and
the digital output of the Cardiac Doppler Ultrasound System 36
(i.e., line 56) to a framing instrument (not shown) which combines
a time code with each image frame, and outputs this combined
digital frame to the magneto-optical disc drive 28. The design of
such framing instruments is well known to one skilled in the art of
digital design for systems that combine blocks of digital
information into an overall frame for further communication. The
advantages of including a time code with each frame may be further
appreciated in connection with the hereinbelow discussion
concerning the correction of the acquired Doppler information.
Preferably, analog waveforms representing the electrocardiogram,
respiration, and electromagnetic blood flow (if available), are
preferably added to the S-VHS video tape by the use of a 3-channel
FM recording adaptor 58 (e.g., Model 3, AR Vetter Co., Rebersburg,
Pa., U.S.A.). This device FM modulates three baseband signals into
a frequency band compatible with an audio channel of the S-VHS
recorder and player 24. This capability permits R-wave gating of
the video images for subsequent image averaging, for example, as
well as for the elimination of respiratory effects, and for the
comparison with direct electromagnetic flow measurement in certain
experimental settings. Preferably, the image information stored on
the magneto-optical drive 23 includes information that indicates
the acquisition time of the image frame with respect to the cardiac
and/or respiratory cycle. For instance, in an embodiment in which
each frame includes a time code, each frame that occurs at a
predetermined time with respect to the cardiac cycle (e.g.,
coincident with the R-wave trigger) may further be provided with
data (e.g., a bit or word) that is indicative thereof. Thus, the
time of occurrence for each digitally stored image frame is known
with respect to the cardiac cycle (and/or respiratory cycle),
thereby facilitating subsequent data analysis.
Referring now to FIG. 3, the 3D-DIPS consists of the components
necessary to read and process the 2D color flow images from both
S-VHS video tape and magneto-optical disk media. A 486-based, 50
MHz data-formatting and color flow correction microcomputer 56
(e.g., Model 450DE, Dell Computer Corporation, Austin, Tex.,
U.S.A.) is used for this function. This microcomputer can read the
TPOMS Output Files for each study, as well as the magneto-optical
disks containing digital color flow images and the structural
images. The TPOMS Output Files can, for example, either be
transported to 3D-DIPS on floppy disks or sent electronically via
Ethernet.
A microcomputer 58, (e.g., Quadra, Model 950, Apple Computer, Inc.,
Cupertino, Calif., U.S.A.) with 132 MBytes of RAM, contains a
high-speed video frame buffer (e.g., Model 2000, Scion Corporation,
Frederick, Md., U.S.A.) with 64 MBytes of dedicated image RAM. This
platform can directly acquire approximately 7 seconds of
640.times.480 line video frames, from a S-VHS video player 60
(e.g., Model AG-7650, Panasonic Corporation, Secaucus, N.J.,
U.S.A.).
The video player 60 can read the SMPTE times codes that are added
to an audio channel of the tape during the recording process, as
described above in connection with the 3D DIAS. In addition, an FM
demodulator 62 is provided for demodulating the waveforms
representing the electrocardiogram, respiration, and
electromagnetic blood flow (if available) that were modulated onto
the audio channel of the videotape. The analog electrocardiogram
signal is provided to generate R-Wave triggering for the video
frame buffer and for an analog-to-digital converter which digitizes
the analog information furnished thereto, and is coupled to a
direct memory access means for storage of the digital information
that has been converted.
It is understood that the 3D DIPS processes the data stored on the
magneto-optical disc which includes separated black-and-white
structural images, and color flow images. The S-VHS tape images are
actually composite images, with color flow data superimposed on the
black-and-white structural data, and therefore they have a number
of inherent disadvantages for providing corrected Doppler images.
Further it is understood that in accordance with a preferred
embodiment of the present invention, since the image data stored on
the magneto-optical disc subsystem are R-wave gated at the time of
acquisition (by the use of an R-wave trigger internal to the
Cardiac Ultrasound System 36), that the digital magneto-optical
disc images are preferably processed as a series of data sets, each
containing a multiplicity of images that were obtained at the same
point in time in the cardiac cycle during successive cardiac
cycles.
It is also appreciated that flexible and efficient data collection
are possible utilizing the 3D-DIAS described. Similarly, the 3D
DIPS also allows an investigator to implement a wide variety of
image processing algorithms, from either analog or digital 2D color
flow Doppler images.
In accordance with the hereinabove described system, a methodology
for practicing the present invention is now further described. The
essential steps include acquiring ultrasound structural (e.g.,
black-and-white or B-mode) and Doppler information, and processing
the structural and Doppler information to provide Doppler data that
is corrected for the Doppler look angle.
Acquiring such data may occur in a variety of ways. In a dynamic
acquisition mode, the ultrasonic probe 12 may be manually scanned
over the region of interest while the 3D DIAS acquires information
at a predetermined rate and with reference to an internal R-wave
trigger. Alternatively, in a static acquisition mode, an operator
may manually position the ultrasonic probe 12 in a plurality of
fixed positions. At each position, the operator manually initiates
the 3D DIAS system to acquire a time series of images for a
predetermined period of time over the cardiac cycle. It may also be
understood, however, that if the blood flow velocity at only a
particular time in the cardiac cycle is desired, in either mode the
system may be adapted to acquire data only at the desired point in
time, since in both instances the triggering mechanism may be based
on the cardiac cycle (e.g., R-wave triggering). During acquisition,
a time series of image frames are stored on the magneto-optical
drive 28. These image frames are stored as pairs of images, one
containing structural information (e.g., black-and-white) and the
other containing Doppler information (e.g., color), as discussed
above. During acquisition, the time code is provided both to the
video recorder 24 and to the microcomputer 26 on a frame by frame
basis. For each frame, the microcomputer 26 also acquires the
position and orientation information from the position/orientation
system 30, and generates and stores the TPOMs file accordingly.
In accordance with an embodiment of the present invention,
processing the structural and Doppler information to provide
corrected Doppler data is performed using the 3D DIPS such that the
structural data is reconstructed to provide flow direction for
points along the longitudinal extent of the vessel. This flow
direction is used in conjunction with the time coded position and
orientation information for the ultrasonic transducer (i.e., TPOMS
file) to correct the acquired Doppler data. As discussed above, it
is preferable to perform this data analysis for separate points in
time with respect to the cardiac cycle since in addition to the
dilation and contraction of the vessel diameter during the cardiac
cycle, there may also be translational motion of the vessel during
the cardiac cycle because of asymmetric mechanical resistance about
the vessel. Similarly, flow velocity is apparently a function of
time in the cardiac cycle, and the corrected flow velocity should
be provided in accordance with the structural reconstruction for
substantially the same point in time. It may be appreciated that
for practical purposes, image acquisition at a typical video frame
rate (e.g., 30 frames per second) is sufficient to consider a
subsequent frames of structural and Doppler information as
occurring at substantially the same time. As mentioned, for a
particular time, a centerline of the vessel is determined according
to the structural (e.g. B-mode) data, and the centerline used to
compute corrected values for the Doppler data. Thus, corrected
Doppler data is provided as a function of time, thereby permitting
accurate measurement and visual representation of blood flow as a
function of time over the cardiac cycle. It is understood that
averaging images which are each identically time locked to the
R-wave and each occur in successive cardiac cycles for a plurality
of cardiac cycles, either before or after correction, may provide
increased signal-to-noise properties.
It may be appreciated that the structural reconstruction may be
performed according to structural data obtained simultaneously with
the Doppler data (through the use of alternating or time
interleaved sequences of structural and Doppler pulses), or in
separate measurements over two different series of cardiac cycles.
In practice, it has been observed that two measurements of
structural data over the cardiac cycle with the ultrasonic probe 12
oriented longitudinally (i.e., position A of FIG. 1) but at
different angles about the vessels longitudinal axis is sufficient
to provide the structural information needed to extract the flow
direction (e.g. centerline axis) along the vessel. Any of myriad 3D
reconstruction techniques may be employed to provide a structural
representation of the vessel, and further, any of myriad numerical
techniques may be employed to select the centerline axis. Also, the
centerline essentially may be manually selected by tracing it on a
display of the vessel in two planes. It is also understood, that
the TPOMs information is also used in the 3D structural
reconstruction in order to ensure proper reconstruction as well as
spatial correspondence with generated color Doppler images.
Moreover, it may be understood, in accordance with discussions
hereinabove, that in an embodiment in which the structural and
Doppler information stored on the magneto-optical drive does not
include R-wave gating information or time code information, that
the video information may be used to determine the time position of
the image frames stored on the magneto-optic disc by comparison
thereof.
In view of the foregoing, it is understood that in accordance with
the time code and position/orientation information, the flow
direction (e.g., preferably the vessel centerline) for the vessel
may be determined. Moreover, this centerline is preferably known as
a function of time. The centerline is then used to correct the
acquired Doppler information.
Different techniques may attempt to reconstruct and quantify 3D
flow profiles similar to the way structures are reconstructed by
using temporal and spatial sequential sets of 2D images, and any
additional information available. Unfortunately, as discussed
above, the Doppler measurement introduces different and additional
measurement errors. Unless the image plane coincides with the
vascular central longitudinal cross-section the measurement of flow
velocity from these images is erroneous. Errors may also be
introduced when the flow direction is not taken into account during
the calculation of flow velocity. The following techniques are
employed to correct the Doppler measurement based on the extracted
centerline.
Referring again to FIG. 1, the correction of flow velocity values
in images of longitudinal cross-sections (i.e., position A),
performed in the image coordinate system ("ICS") is conducted as
follows. For each pixel p.sub.L in location (h.sub.i, v.sub.j)
within the image, which contains flow velocity values, a correction
is made in the ICS, according to the equation: ##EQU1## where
.delta. is the angle between the flow orientation and the
ultrasound probe orientation.
For images acquired with the Linear Array probe, with the beam
steered sideways (angle .PSI. up to +/-60.degree.), then
##EQU2##
Analogously, correction of flow velocity values in images of
transverse cross-sections (i.e., position B), performed in the
reference coordinate system ("RCS"), are conducted as follows. For
each pixel P.sub.T in location (h.sub.i, v.sub.j) within the image,
which contains flow velocity values different than zero flow, a
correction is made in the RCS, according to the equation: ##EQU3##
where
Matrices M.sub.L and M.sub.T are calculated from the Euler angles
of the Ultrasound Probe orientations with respect to the X.sub.RCS,
Y.sub.RCS, Z.sub.RCS axes of the Reference Coordinate System as
acquired from the position/orientation measurement system 32.
For the longitudinal and transverse cross-sections, respectively,
M.sub.L and M.sub.T are defined as: ##EQU4## where matrix M.sub.LR
=M.sub.L .times.M.sub.R, and where M.sub.R is Rotational matrix,
which transfers the flow orientation from the image plane to the
Receiver's coordinate system.
Assuming the flow orientation within the Receiver's x-y image
plane, the Euler angles for x, y are both zero, and z=.delta.,
which results in M.sub.R defined as: ##EQU5##
As a result of this methodology, the acquired Doppler information
is transformed into corrected Doppler information that may be
employed to accurately represent the flow velocity in the vessel as
a function of space and time. As evinced by the foregoing
description, currently available components may be interconnected
to provide a system for practicing this methodology. A preferred
implementation of corrected color Doppler imaging in accordance
with the present invention should include direct logging of time
and position information with the digitally stored image data.
Thus, as illustrated through the preferred embodiment and the
foregoing example, and as understood by further practicing the
present invention, many advantages and attendant advantages are
provided by the present invention. The method developed produces an
accurate determination of blood flow velocity non-invasively. This
allows not only improved calculation of velocity profiles, flow
volume and resistances, but also estimations of pressures across
valve orifices and stenotic arteries. The calculation of flow
velocity near the arterial walls (currently not computable by most
systems), will allow the estimation of shear stress and evaluation
of possible future damage to the endothelial surface. Similarly,
applying the same procedure to the measurement of the velocity
profile across artificial cardiac valves, may provide good
estimates of their contribution to blood clot formation. The method
provides an important clinical tool for screening and evaluation of
vascular pathologies. With further development of Transesophageal
Echocardiography, the method may be adapted for evaluating coronary
artery stenosis. It may also be appreciated that the present
invention provides for free space movement of the ultrasound probe.
Thus, the method and system provides for numerous new modalities
for medical diagnosis and care.
Although the above description provides many specificities, these
enabling details should not be construed as limiting the scope of
the invention, and it will be readily understood by those persons
skilled in the art that the present invention is susceptible to
many modifications, adaptations, and equivalent implementations
without departing from this scope. For example, as discussed above,
although the invention is described by way of example as including
a separate acquisition and processing systems, and an embodiment is
disclosed in accordance with convectional, commercially available
equipment, one skilled in the art recognizes that the overall
invented method and system may be included as a mode of a
conventional ultrasound instrument and may be implemented with
dedicated hardware and software. Further, image processing may be
simplified by storing all pertinent information digitally within
the image frames. Moreover, one recognizes that many image
processing and data representation applications and manipulations
may be applied to the corrected Doppler data.
These and other changes can be made without departing from the
spirit and the scope of the invention and without diminishing its
attendant advantages. It is therefore intended that the present
invention is not limited to the disclosed embodiments but should be
defined in accordance with the claims which follow.
* * * * *